AGE OF THE MAGMATIC AND METAMORPHIC PROCESSES IN THE VODLOZERO COMPLEX, BALTIC SHIELD: AN ION MICROPROBE (SHRIMP II) U-TH-PB ISOTOPIC STUDY OF ZIRCONS

Abstract

Trondhjemite gneisses in Central Finland with 3.7 Ga-old zircon cores are the only old rocks found as of yet in the Baltic shield. The areas promising for the discovery of ancient zircons are located in the Vodlozero domain [2]. The Vodlozero gneisses were previously dated by the conventional U–Pb zircon method at 3115 ± 17 Ma (two-point isochron on early zircons) and 2873 ± 15 Ma (late zircon), while K–Ar and Pb–Pb dates on amphiboles and plagioclases varied from 2850 to 2650 Ma, reflecting later superimposed events. No reliable ages were obtained by dating of early zircons on SHRIMP I in Canberra. The dates thus obtained were discordant, which is difficult to interpret during ion microprobe zircon dating.